The invention relates generally to an echo sounding apparatus and more particularly to circuitry for enhancing the representation of echo signals recorded on electrically sensitive paper.
One recording medium which has been found quite suitable and is commonly used for recording representations of echo sounding signals is electrically sensitive paper. The present invention is described in a system for achieving enhanced recording of echo sounding signals on such paper. However, it is also useful for achieving enhanced representations of signals having a wide dynamic range on various other media.
Electrically sensitive paper typically comprises a conductive layer of carbon covered by an opaque layer of paper which, during the recording operation, is burned away to varying degrees to variably expose the black carbon layer underneath. It is known that the dynamic range of echo sounding signals is substantially larger than the range of variation which can be distinguishably recorded on electrically sensitive paper. More particularly, echo sounding signals commonly cover a range of about 40 dB which corresponds to a ratio of 1:100. By way of comparison, the range of variation which can be readily distinguished on electrically sensitive paper is only about 20 dB, or a ratio of 1:10.
Dynamic compression of the signals for reconciling these ranges of change is not suitable since, it would also result in a substantial reduction in contrast of the record. One solution to this problem, is shown in U.S. Pat. No. 3,350,682 issued to the present application on Oct. 31, 1967. This patent discloses echo sounding apparatus in which amplifier gain is controlled both by negative feedback comprising the output signal from which AC-components have been filtered and unfiltered positive feedback. Such an arrangement effectively provides higher amplification of rapidly changing echo signals than DC signals or slowly changing signals. The negative feedback prevents the recorder from operating in a saturated mode. This combined use of positive and negative feedback results in an increase in the contrast of the record. Accordingly, it permits enhanced recording of objects closely above or below the sea bottom, whose signals would be almost indistinguishable in the stronger return signals from the interface between the sea water and sea bottom. In addition, the disclosed technique is useful in reducing the effects of high attenuation of the echo signal within the sea water medium so as to permit the detection of weak echos from objects of interest below and above the sea bottom.
The applicant's U.S. Pat. No. 3,332,056 issued on July 18, 1967 discloses a circuit for achieving a frequency-dependent increase in the contrast of recorded echo signals. In this circuit echo signals are separated into higher frequency and lower frequency portions on the basis that echoes from fishes and small structures, because of their smaller dimensions, include more pulse-like and therefore higher frequency portions than echos such as those produced by the sea bottom. If the higher frequency portions of the signals are filtered out by means of a frequency-dependent network and are amplified more than the lower frequency portions, the representation of fishes and small structures is improved. The circuit comprises two low pass filters with substantially different time constants, the output signals of which are combined so that the low frequency portions are at least partially suppressed and the high frequency portions appear more prominent.
It is the object of the present invention to improve the readability of a record of echo signals such that even weakly reflecting objects and structures can easily be noticed and are not covered by the echoes from strongly reflecting objects, in particular by the echo from the sea bottom.